During the summer solstice, the Sun shines most directly on the Tropic of Cancer, 23.5 degrees north of the equator, providing the Northern Hemisphere with the most direct energy on Earth. At this time of year, the Sun is over the horizon for half of the world's population and may even be below the horizon for the other half. The word "summer" comes from the Old English sūmhræft, which means "sunshine."
In contrast, at the winter solstice, the Sun is most directly overhead at the North Pole, where it remains throughout the month of January. At this time of year, the Sun is above the horizon all day long for those living at the South Pole. The word "winter" comes from the Old English wintre, which means "the season following spring."
Overall, the Sun is about 15 degrees higher in the sky in the summer than in the winter.
The angle between the Sun and the Earth changes throughout the year. In February when the Sun is at its highest position in the sky, it is 90 degrees to the Earth. By June when the Sun is at its lowest position, it is 60 degrees.
This is called the solar cycle and it affects everything from ice caps to ocean levels.
The sun appears directly above the Tropic of Cancer, the latitude line at 23.5 degrees North, during the summer solstice in the Northern Hemisphere. (That's the furthest north you can go while still seeing the sun straight overhead.) At this time of year, the sun is at its highest position in the sky and causes days to be longest. The angle between the earth and the sun is greatest at the summer solstice, which means that day length is also greatest. After the summer solstice, the sun begins to move southward, causing days to get shorter and nights longer until the winter solstice in December.
At the winter solstice, the sun is again directly over the Tropic of Capricorn, this time at 35 degrees North, and it remains there all year long. Because the angle between the earth and the sun is smallest at the winter solstice, night length is also shortest. As the year progresses, the sun moves farther away from the equator, causing days to get longer and nights to get shorter.
During the summer months, more of the Earth is illuminated by sunlight than at any other time of the year. This is because the center of our planet is constantly facing toward the sun, so during the summer half of the year, more of the surface is exposed to light than at any other time of the year.
Seasonal Changes The Tropic of Cancer, as shown in the link, is the latitude at which the Sun is directly overhead on the summer solstice. The Sun is now at a declination of 23 degrees north of the celestial equator, and the equivalent latitude on Earth is also 23 degrees north of the equator. At this time every year, the sun rises in the east and sets in the west at virtually the same hour everywhere on Earth that it has risen and set at the same time every day for many years (unless there are major solar eclipses).
The line of equal sunset and sunrise hours marks the location where days are of equal length. Because we live near the Equator, we experience only 12 hours of daylight per day in the summer and 12 hours of darkness during the winter months. But if we were living at the North Pole, where the sun is directly over head for all 24 hours of the day, there would be no night during the summer months.
It is because of this reason that the Arctic Circle exists: to mark the point where daytime and nighttime occur simultaneously on the globe. At the Arctic Circle, the sun is up for half of the year and down for the other half. Locations further south have their nights longer than their days; at the South Pole, for example, the sun is up for all but a few minutes each day and does not rise at all during the winter months.
During the winter solstice, the Sun shines most directly on the Tropic of Capricorn, 23.5 degrees south of the equator, providing the Southern Hemisphere with the most direct energy on Earth. As a result, the temperature difference between the equator and the pole is greatest at this time, with the Arctic experiencing temperatures as low as -45 degrees Celsius and the Antarctic frozen over completely.
The Northern Hemisphere experiences winter for half of the year and summer for the other half, but within each season, the Sun's rays are either reflected away from Earth or absorbed by clouds and water vapor before they can reach it. The amount of solar energy that reaches the surface of Earth decreases during winter because much of it is reflected back out to space.
Even though the Sun is not visible during the night hours of winter, it is still shining at an angle far enough toward the Earth that some of its light gets through. The part of the Sun's beam that makes it past the horizon is called a lunar crescent. Lunar crescents are useful for navigating at sea, since they allow you to see where you have been even if it is dark outside your ship's lantern. Landmarks seen in the lunar crescent will appear again in the morning when the Sun rises.
At the equator, the sun's rays reach the Earth's surface the most directly. The sun's energy is most concentrated in the lowest latitudes. The highest latitudes receive the least amount of sunlight. Because of the Earth's tilt, various places receive varying quantities of solar radiation. For example, the far north experiences winter when much of the land is covered in ice or snow. The far south experiences summer when the entire world is engulfed in a relentless heat wave.
The amount of solar energy that reaches the surface of the Earth varies by season and latitude. At mid-latitudes, such as in parts of the United States, there is a direct correlation between solar altitude and daytime temperature. As solar altitude increases, so does ambient air temperature. This is because more ultraviolet (UV) radiation is reaching the ground at high altitudes than at low altitudes. Due to the absorption of UV radiation by the atmosphere, more solar energy reaches the ground at high elevations than low ones. Climate zones are determined by which hemisphere has the sunniest climate. Tropical regions are sunny throughout the year, while temperate regions experience more clouds and rain during spring and summer.
In polar regions, the sun never sets and days are either completely dark or fully light for many months at a time. The amount of solar energy that reaches the surface of the Earth here is very low because the angle between the Earth and the sun is nearly 90 degrees.